In one case of a small drop of mercury one force of gravity is small and force of surface tension plays a vital role. Therefore, the free surface of drop tends to have minimum surface area for given volume the sphere has minimum area. Hence the small drops are of spherical shape.
In the case of large mercury drop, the gravitational pull becomes more effective than the surface tension and exerts downward pull on the drop so that its centre of gravity may lied at one lowest possible position. Hence, the large drop of mercury becomes elliptical or flat in shape.